Methods and systems for providing efficient operation of multiple modes in a wlan system

ABSTRACT

A method and apparatus that applies medium access control (MAC) transmission opportunity (TXOP) protection for multiple mode operation in a WLAN system. In particular, MAC mechanisms are defined to support multiple mode CTS frames, and multiple mode CF-End frames sent by the AP, each in a format appropriate for the corresponding mode which may also apply to a single mode. MAC mechanisms permit truncation of TXOP duration for releasing the unused portion of the TXOP when no further data for transmission is available. Release of unused protected TXOP is possible for both protected AP transmissions and STA transmissions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/681,897 filed on Nov. 20, 2012, which issued on Dec. 9, 2014 as U.S.Pat. No. 8,909,286, which claims the benefit of U.S. patent applicationSer. No. 13/218,919 filed on Aug. 26, 2011, which issued on Jan. 22,2013 as U.S. Pat. No. 8,359,065, which claims the benefit of U.S. patentapplication Ser. No. 11/618,328 filed on Dec. 29, 2006, which issued onSep. 6, 2011 as U.S. Pat. No. 8,014,818, which claims the benefit ofU.S. Provisional Application No. 60/756,457 filed on Jan. 4, 2006 andU.S. Provisional Application No. 60/796,176 filed on Apr. 29, 2006, thecontents of which are hereby incorporated by reference.

FIELD OF INVENTION

The present invention generally relates to wireless local area networks(WLANs). More particularly it enhances operation of STAs in multiplemode deployment in the same coverage area.

BACKGROUND

Currently, various proposals are being presented and discussed for the802.11n extension to the 802.11 WLAN standard, which will allow forhigher throughput WLAN devices. These proposals come from variouswireless consortiums that include EWC, the Joint Proposal and WWiSE. Thefollowing describes aspects of these proposals relevant to the presentinvention.

FIG. 1 shows a Clear to Send (CTS) frame as a MAC control frame asdefined in the 802.11 standard. The receiver address (RA) of the CTSframe is copied from the transmitter address (TA) field of theimmediately previous Request to Send (RTS) frame to which the CTS is aresponse. The duration value is the value obtained from the Durationfield of the immediately previous RTS frame, minus the time, required totransmit the CTS frame and its short inter-frame spacing (SIFS)interval. If the calculated duration includes a fractional microsecond,that value is rounded up to the next higher integer.

The CTS frame need not always follow a RTS frame as described in the802.11e standard (section 7.2.1.2). It can be the first frame in anexchange and used for setting the Network Allocation Vector (NAV) forMAC level protection for the transmission to follow. When the CTS frameis sent as the first frame by the initiating station of an exchange, theCTS may be addressed to itself and is referred to as CTS-to-self.

FIG. 2 shows a Contention Free End (CF-End) frame, which is a MACcontrol frame that may be sent by the AP as a broadcast frame to resetthe NAVs of all stations in the system and is described in the 802.11standard. A station receiving a CF-End frame with the basic service setID (BSSID) of the BSS, to which the station is associated, will resetits NAV value to 0. This resets any medium protection/reservationcurrently existing. The Duration field is set to 0. As shown in FIG. 2,the BSSID is the address of the STA contained in the AP. The RA is thebroadcast group address. The FCS is the frame check sequence.

In 802.11n, proposals were made to implement support for Extended Rangeusing a different physical layer (PHY) modulation scheme than that usedfor Normal Range, essentially creating two modes of operation. ExtendedRange STAs transmit and receive using Space Time Block Code (STBC) PHYmodulation, whereas Normal range STAs transmit and receive using anon-STBC PHY modulation. In a Joint Proposal contribution to 802.11n, anapproach is described for an AP to support a network of STAs operatingin a dual mode, where the two modes are Extended Range and Normal Range.Secondary beacon and Dual CTS method together are used to supportExtended Range in addition to Normal Range, A secondary beacon istransmitted with a secondary beacon bit set in the beacon to letstations know that the target beacon transmission time (TBTT) for thisbeacon has an offset. In the Dual CTS protection, stations start a TXOPwith an RTS directed at the AP, and the AP responds with a first andsecond CTS separated by a point control function inter-frame spacing(PIFS). When dual CTS protection enabled, the AP should protect STBCTXOPs with a non-STBC CTS and non-STBC TXOPs with an STBC CTS. Theprotection frames shall set a NAV for the entire TXOP. STBC controlframes shall be used in response to STBC frames if the Dual CTSprotection bit is set. Non-STBC control frames shall be used otherwise.PIFS is used as the interval to separate the dual CTS for non-STBC RTS.

FIG. 3 shows a diagram from the WWiSE proposal presentation document onthe self-managed Extended Range protection. Examples of signaling fordual mode protection of normal range (NR) and extended range (ER)stations are shown. Signal sequences 301-305 relate to enhanceddistributed coordination function (DCF) channel access (EDCA) and signalsequence 306 relates to a HCF controlled channel access (HCCA) format.The AP protects TXOP for the NR STA and ER STA using signal sequences301-302 respectively. The ER STA protects its TXOP in signal sequence303. A signal sequence for an 11n NR STA is represented by signalsequence 304, and one for a legacy NR STA is represented by signalsequence 305. In signal sequence 306, the AP protects a TXOP for the STAusing HCCA format. As shown, the AP sends either a CTS in response to anRTS from a particular stations and in the mode as used by the stationthat sent the RTS, or a CTS-to-self signal in the mode other than thatof the RTS-sending station.

FIG. 4 shows a new HT information element according to the WWiSEproposed Extended Range. The AP signals new HT information elements inmanagement frames such as beacon, probe response etc. to manage the BSS(for example to support Extended Range). The new HT information elementsmay also be present in all beacons and probe responses transmitted by astation in IBSS mode. The HT information elements contain fields such asSecondary beacon, dual STBC/CTS protection, etc. as shown in FIG. 4.According to the Joint Proposal, the length is not fixed and the sizedepends on the number of fields that are included. The fields shall bein the order as shown in FIG. 4, with any new fields appearing at theend of the existing fields. Any fields unknown to the STA shall beignored.

According to the Joint Proposal specification, and the EWCspecification, following are some definitions related to Power SaveMulti-Poll (PSMP) feature. A Power Save Multi-Poll (PSMP) is a MAC framethat provides time schedule to be used by the PSMP transmitter and PSMPreceivers. The time schedule begins immediately subsequent to thetransmission of the PSMP frame. A downlink transmission (DLT) is aperiod of time described by a PSMP frame, which is intended to be usedfor the reception of frames by PSMP receivers. An uplink transmission(ULT) is a period of time described by a PSMP frame, which is intendedto be used for the transmission of frames by a PSMP receiver.

FIGS. 5 and 6 show PSMP information element formats according to the EWCMAC specification. FIG. 5 shows a PSMP parameter set format in which thePSMP is of type/subtype Management Action Frame and broadcast addresstype. The PSMP parameter set is used to describe the DLT and ULT whichimmediately follows the PSMP frame. FIG. 6 shows the STA Infoinformation element format details such as Traffic (flow) ID, STA ID,DLT offset and duration, ULT offset and duration.

FIG. 7 shows the PSMP sequence consisting of a DLT phase followed by aULT phase. Multi TID Block ACK (MTBA) is used to send Block ACK formultiple TID flows.

A need exists to extend a dual mode protection to supporting multiplemode operation. The current art is not robust and efficient in mediumusage because it does not provide a mechanism to recover any unusedtransmission opportunity (TXOP) duration protected by the dual CTStransmission. Under the current art scheme, if the STA runs out of datato transmit during the protected TXOP, the medium is wasted for theremainder of the TXOP. A need exists to provide MAC signaling torelinquish the remaining unused TXOP to the system.

A need also exists for the PSMP sequence to operate in a multiple modesystem in a bandwidth efficient manner. The 802.11n specificationcontains inconsistencies with respect to allowing only ACK/MTBA in ULTand no data for unscheduled PSMP. Also, there is no guidance fortruncation of TXOP under dual CTS protection for STAs that are not ableto interpret the CF-End frame.

SUMMARY

A first embodiment is a method and system for extending a specific (STBCand non-STBC) dual mode operation in a WLAN system to a more generalmultiple mode operation. A second embodiment is a method and system forenhancing the MAC protection mechanisms in multiple mode operation, inparticular, mechanisms to support a multiple CF-End (each in a formatappropriate for the corresponding mode) frame sequence sent by the AP toenable efficient medium utilization which may also apply to a singlemode. A third embodiment is a method and system for enhancing PSMPsequences in multiple mode operation.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description of a preferred embodiment, given by way of exampleand to be understood in conjunction with the accompanying drawingswherein:

FIG. 1 shows a CTS frame according to the 802.11 Standard;

FIG. 2 shows a CF-End frame format according to the 802.11 Standard;

FIG. 3 shows a signaling diagram for self-managed extended rangeprotection according to WWiSE;

FIG. 4 shows a management frame HT information element format;

FIG. 5 shows a PSMP parameter set format;

FIG. 6 shows a PSMP STA Info information element format;

FIG. 7 shows the PSMP sequence consisting of a DLT phase followed by aULT phase;

FIG. 8 shows an exemplary wireless LAN operating in multiple mode;

FIG. 9 shows a primary beacon format and a secondary beacon format thatincludes primary and secondary beacon ID fields;

FIG. 10 shows a management frame HT information element format thatincludes primary and secondary beacon IDs;

FIG. 11 shows a frame transmission of a STA using protecting TXOP for aspecific mode format;

FIG. 12 shows a frame transmission of an AP protecting TXOP using EDCA;

FIG. 13 shows a frame transmission of an AP protecting TXOP using HCCA;

FIG. 14 shows a frame transmission sequence of an STA releasing unusedTXOP;

FIG. 15 shows a frame transmission sequence of an AP releasing unusedTXOP using EDCA;

FIG. 16 shows a frame transmission sequence of an STA releasing unusedTXOP using HCCA; and

FIG. 17 shows a multiple mode PSMP frame sequence.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, the terminology “station” or “STA” includes but is notlimited to a wireless transmit/receive unit (WTRU), a user equipment(UE), a mobile station, a fixed or mobile subscriber unit, a pager, acellular telephone, a personal digital assistant (PDA), a computer, orany other type of user device capable of operating in a wirelessenvironment. When referred to hereafter, the terminology “base station”includes but is not limited to a Node-B, a site controller, an accesspoint (AP), or any other type of interfacing device capable of operatingin a wireless environment.

Hereafter for the purposes of describing the invention, “mode” is usedto refer to the specific network link, below the MAC Layer, used forcommunication (transmission and reception) such as the PHY layer,channel interface, channel bandwidth (e.g., 20 MHz versus 40 MHz) andphysical communication channel. It should be noted that STAs indifferent modes may not typically operate efficiently together in a BSScoverage area, unless controlled and protected by MAC layer mechanisms.The present invention relates to a multiple mode system (e.g., BSS)where STAs transmit and receive in multiple modes (more than one) in thesame coverage area.

FIG. 8 shows an exemplary wireless LAN, comprising an AP, and a STA1operating in a Mode 1 operation, and a STA2 operating in a Mode 2operation. For simplicity, the preferred embodiments are described inthe context of two modes, Mode 1 and Mode 2. However, the presentinvention may be extended to multiple mode operation that includesadditional modes beyond two.

The following describes three preferred embodiments of the presentinvention. The first is a method and system for enhancing a specific(space time block coding (STBC) and non-STBC) dual mode operation in aWLAN system to a more general multi-mode operation. The secondembodiment is a method and system for enhancing the MAC protectionmechanisms in multiple mode operation, in particular, mechanisms tosupport a multiple CF-End (each in a format appropriate for thecorresponding mode) frame sequence sent by the AP to enable efficientmedium utilization which also applies to a single mode as a trivialcase. The third embodiment describes a method and system for enhancingPSMP sequences in multiple mode operation.

The first embodiment concerns defining MAC mechanisms to supportmultiple mode operation. Examples of applications for multiple modeoperation include: (1) legacy systems, (2) devices supporting a newmodulation set, (3) devices which may be in a transition mode (newmodulation set) before switching networks, (4) mesh networks supportingmultiple modes, and (5) devices operating on more than one frequencyband/channel.

In accordance with the first preferred embodiment, the AP supportsmultiple mode operation using two main MAC mechanisms: 1) by sending abeacon/secondary beacon followed by multicast/broadcast data for eachmode supported; and 2) by supporting the sending of multiple CTS frames,each corresponding to one of the multiple modes that are supported. Thechallenge for the multiple mode protection is that the CTS protectionframes must be interpreted in the mode format (modulation, linkconfiguration, etc.) by each of the two communicating entities. Thus, ifa STA is using a specific mode format, then the CTS protection framemust be sent and received in that specific format to allow recognitionby the STA.

FIG. 9 shows a diagram that is representative of a preferred set offrame formats according to the above MAC mechanisms of the AP. A primarymode frame comprises a primary beacon 901 followed by themulticast/broadcast data 905. The primary beacon includes an HTInformation Element 903. After a defined offset period, a secondary modeframe is sent that includes a secondary beacon 902 with its HTInformation Element 904, followed by multicast/broadcast data 906. Withreference to FIG. 8, the primary beacon serves STA1 on mode 1(non-STBC). The secondary beacon serves STA2 which uses mode 2 (STBC).While for the purpose of example, here, mode 1 and mode 2 have beenaligned with primary beacon and secondary beacon respectively,alternatively, the primary beacon can serve mode 2 and the secondarybeacon can serve mode 1, depending on system parameters. Returning toour example, generally, the primary beacon will serve all stations usingmode 1, and the secondary beacon will service all stations using mode 2.For multiple mode operation, additional secondary beacons will serveeach of the modes used in the system, respectively.

During the multiple mode operation, the AP sends a beacon/secondarybeacon and multicast/broadcast traffic in a format suitable for eachmode supported by the system. In a multiple mode system, one of theseveral beacons transmitted (corresponding to the several modes) isidentified as the primary beacon 901. Each secondary beacon 902 may betransmitted with a time offset (with reference to the primary beacon 901or any other time reference). The time offset may be determined based onsystem considerations. The time offset may be a configurable systemparameter that could be changed dynamically by the AP. A timingsynchronization function (TSF) timestamp of the secondary beacon 902shall be the actual timestamp. All other fields in the secondary beacon902 are preferably identical to the corresponding fields in the primarybeacon 901. The multicast/broadcast data 906 transmitted after thesecondary beacon 902 is preferably identical to the multicast/broadcastdata 905 sent after the primary beacon 901. Based on systemconsiderations, each secondary beacon 902 includes extra fields and dataunique to its mode. Also based on system considerations, each mode mayhave extra multicast/broadcast fields and data unique to its mode.

FIG. 10 shows the preferred format for an HT Information element 1000,corresponding to HT information elements 903, 904. The HT Informationelement 1000 comprises the following fields: element ID 1001, length1002, control channel ID 1003, extension channel offset 1004,recommended transmission width set 1005, RIFS mode 1006, controlledaccess only 1007, service interval granularity 1008, operating mode1009, Basic STBC MCS 1011, L-SIG protection allowed 1013 and Basic MCSset 1016. These fields correspond with the proposed management HTinformation element format shown in FIG. 4. In accordance with thepresent invention, a multiple mode protection field 1012 and beacon IDfield 1014 are included to support multiple mode. As an example for dualmode, the Beacon ID field 1014 may be one bit, where if the HT infoelement has a value of 0, it is primary beacon, and if the value equals1, then it is a secondary beacon. For multiple mode, however, a singlebit info element is extended to a size adequate for identification ofall existing modes besides the primary mode. As shown in FIG. 10, thebeacon ID field 1014 is tagged by bits B9-Bk where k is selected basedon the number of supported modes. For example, in a system using 16modes, a beacon ID field of 4 bits (B9-B12, k=12) is selected.

FIG. 11 shows an example signaling diagram 1100 for a multiple modesystem using n modes that includes the AP and the station STA2, which isoperating in Mode 2 and protecting a TXOP. An indication is provided bythe AP that multiple mode TXOP protection is supported by the system.The preferred mechanism for this indication is for the AP to signal amultiple CTS protection field/bit in the new HT information elementMultiple Mode Protection 1012 as shown in FIG. 10. When the multiple CTSprotection field/bit is set by the AP and received by the station STA2,a TXOP is started by the station STA2 with a request to send (RTS) frame1101 in Mode 2 is transmitted to the AP. The response from the AP is tosend multiple CTS and CTS-to-self frames 1102-1105 in formatscorresponding to the modes, for example, modulation, link configuration,etc., so that stations operating in the other modes will be notifiedthat a TXOP has been reserved/protected for Mode 2 stations, such asSTA2.

As shown in FIG. 11, the AP transmits a CTS frame 1102 in the mode beingused for the TXOP being protected by the STA. Here, the STA is thestation STA2 which initiated the TXOP is operating in Mode 2, and theposition of the Mode 2 CTS frame 1102 in the multiple CTS frame responsefrom the AP is first. Alternatively, the position for this mode's CTSframe may be last, or as determined by the system and based on priorityassigned to modes. The AP also sends multiple CTS-to-Self frames1103-1105 in all modes except in the mode being used for the TXOP beingprotected by the STA, i.e., CTS-to-Self Mode 1, CTS-to-Self Mode 3 . . .CTS-to Self Mode n. The relative order of these CTS-to-Self frames canbe arbitrary or determined based on system and implementationconsiderations and based on priority assigned to modes.

The multiple CTS/CTS-to-Self frames 1102-1105 are separated by a PIFS,SIFS (as shown) or other time duration, such as Reduced Inter FrameSpacing (RIFS), as determined based on other system factors. Once themultiple CTS/CTS-to-self frames 1102-1105 have been completely sent, theMode 2 TXOP 1106 commences.

The multiple CTS/CTS-to-Self frames sent by the AP in response to theRTS frame applies to the following cases. Where a BSS with an AP iscommunicating in a multiple mode operation using multiple CTS signals,the response by each of the STAs is with a single CTS frame in theformat corresponding to its mode of operation. Alternatively, each STAcan be allowed to respond with multiple CTS frames, which isparticularly useful in an independent basic service set (IBSS) (i.e.,where there is no AP and all stations are peers) or a mesh scenario. Insuch a case, a selected STA plays the role of an AP by sending themultiple CTS frames. Otherwise, coordinating the CTS response fromseveral stations could be difficult.

FIG. 12 shows an example signaling diagram 1200 of the AP protecting aMode 2 TXOP using EDCA, which corresponds to the dual protection signalsequence 301 of FIG. 3. Here, the AP initiates a Mode 2 TXOP for itself,beginning with multiple CTS-to-self frames 1201-1203 in all modes exceptfor Mode 2. Again, as in FIG. 11, the sequence of the multiple modeCTS-to-self frames may be arbitrary or determined based on system andimplementation considerations and based on priority assigned to modes.Next, the AP sends a Mode 2 RTS frame 1204, which contains specific STAaddress information, addressed particularly to STA2 for this example. Inresponse, STA2 sends a Mode 2 CTS frame 1205, which allows the Mode 2TXOP frame 1206 from the AP to commence, where AP transmits data in Mode2.

FIG. 13 shows an example signaling diagram 1300 of the AP protecting aTXOP for a Mode 2 STA using HCCA, which corresponds to the dualprotection signal sequence 306 of FIG. 3. When the multiple CTSprotection field/bit 1012 is set and sent by the AP, the AP protects aTXOP in a given mode with multiple CTS-to-self frames 1307-1310 sent informats corresponding to the modes, e.g., modulation, linkconfiguration, etc., except in the mode being used for the TXOP beingprotected by the AP, which is Mode 2 in this example. The order of themultiple CTS-to-Self frames 1307-1310 corresponding to the multiplemodes may be arbitrary or determined based on system and implementationconsiderations and based on priority assigned to modes. The multipleCTS-to-Self frames 1307-1310 may be separated by SIFS (as shown), PIFSor other time duration, such as RIFS, as determined based on othersystem factors.

As shown in FIG. 13 the multiple CTS-to-self frames 1307-1310 arefollowed by a CF-poll frame 1311 according to HCCA protocol, sent in themode being used for the TXOP, after a SIFS, PIFS or other time durationsuch as RIFS, as determined based on other system factors. Here, theTXOP 1312 is for Mode 2, thus the CF-Poll frame 1311 is in Mode 2.

Under this multiple mode TXOP protection embodiment, where the TXOP fora STA is protected, the STA must wait before it starts its transmissionsuntil the multiple CTS or CTS-to-Self frames from the AP aretransmitted. To achieve this, the following preferable procedures areobserved either individually or in various combinations. Preferably, theamount of time needed by the AP to transmit the multiple CTS/CTS-to-Selfframes will be made known to the STAs in the system. An example of onepossible approach is to include this information in a field of the newHT information element 1000 sent by the AP. Alternatively, a stationwill not start transmitting before it receives a CTS response to itsRTS, and if such CTS response comes last, then no explicit time needs tobe communicated beforehand. Another approach is to rely on carriersensing before transmitting, i.e. even after receiving a CTS, the STAwould have to wait if the medium is still occupied by CTS frames ofother modes.

Alternatively, if all STAs are capable of transmitting and receiving ona single common mode format, even if they normally communicate in aspecific mode, that common mode format is preferably used for sendingprotection control frames such as RTS and CTS. The modulation used forsending control frames is typically the basic rate in a given mode. Thehigher rates in each mode are used for data transmission. It isconceivable for a STA to support basic rates in all modes and higherrates only in one preferred/specific mode. In this case, a single RTSframe and single CTS frame being exchanged between two communicatingdevices in that common format is sufficient to establish protection inmultiple mode system operation.

In all of the above protection mechanisms for multiple mode operation,the protection frames that are used (i.e., RTS, CTS) preferably set aNAV for the entire TXOP being protected.

A second preferred embodiment of the present invention provides MACmechanisms to support efficient usage of the medium in multiple modeoperation by releasing unused portions of the protected TXOP. FIGS.14-16 show examples signal sequences of how the multiple CF-End frametransmission may be used to release unused TXOP so as to enhance mediumusage efficiency.

FIG. 14 shows an example of the STA releasing unused TXOP in Mode 2. Asin the signal sequence shown in FIG. 11, the STA2 sends a Mode 2 RTS1401, the AP responds with multiple CTS/CTS-to-self frames 1402-1405,and the STA2's TXOP commences in Mode 2. In this embodiment however,STA2 recognizes that no further data is available for transmission priorto the end of the TXOP frame 1406. STA2 then sends a single End of Dataframe 1416, which can be in the format of a CF-End frame. The APresponds with multiple CF-End frames 1407-1409 in all modes. Once allCF-end frames are sent, the unused portion of the TXOP frame 1406 isreleased to the medium, and a new TXOP protection process can commence,initiated by another station or the AP for its own transmission on themedium.

FIG. 15 shows an example signal sequence of the AP releasing unused TXOPduring EDCA in Mode 2 as an extension of the signal sequence shown inFIG. 12. The AP sends multiple mode CTS-to-self frames 1521-1523,followed by a mode 2 RTS frame 1524 to request a Mode 2 TXOP protection.STA2 responds with a Mode 2 CTS frame 1525, clearing the way for the APto commence its TXOP frame 1506 in Mode 2. During the TXOP frame 1506,the AP recognizes that there is no more data to transmit, so it sends anEnd of Data frame 1526, which can be in the format of a CF-End frame.The AP then sends multiple CF-End frames 1527-1529 in all modes tonotify all STAs that the AP has completed its Mode 2 transmission in thecurrent TXOP frame 1506. The TXOP frame 1506 is then truncated and theunused remainder of the TXOP frame 1506 is then released for access toanother STA or the AP in a different mode. Protection of the releasedTXOP follows the multiple mode procedures described above.

FIG. 16 shows an example signal sequence of the STA releasing unusedTXOP during HCCA in Mode 2 as an extension of the signal sequence shownin FIG. 13. AP sends multiple CTS-to-self frames 1601-1604 in all modesexcept for the mode of the TXOP protection, which is Mode 2 in thisexample. The Mode 2 CF-Poll frame 1605 is sent and the Mode 2 TXOP frame1606 for STA2 commences. During the TXOP frame 1606, STA2 recognizesthat its transmission data has been depleted, so it sends the End ofData frame 1612. The AP notifies the other STAs in all modes usingmultiple CF-End frames in the respective modes. The TXOP remainder isthen released.

As shown in FIGS. 14-16, the AP sequentially sends multiple CF-Endframes within MAC protocol data units (MPDUs) with transmission formats(modulation, link configuration, etc.) corresponding to the modessupported by the AP. A time gap of SIFS (or other time duration asdetermined based on other system factors) is included between the CF-Endframes.

The following are additional examples of conditional cases (individuallyor in combination), where this embodiment for releasing protected TXOPis applicable:

-   -   a. After receiving an End-of-Data MAC signal from the STA, as        shown in FIG. 14, (or for example a QoS-NULL frame with an ACK        response from AP) which started the TXOP;    -   b. After receiving an End-of-Data MAC signal from the STA (or        for example a QoS-NULL frame with an ACK response from AP) which        started the TXOP and the AP not having any data to send;    -   c. If the station which started the TXOP just stops sending        data;    -   d. If the station which started the TXOP just stops sending data        and the AP detects this by some means (such as Carrier Sensing)        and the AP does not have any data to send;    -   e. After any medium recovery procedure; i.e. AP just recovered        the medium, and can send CF-End frames to allow stations to        access the medium;    -   f. If the AP initiated the TXOP and is done with downlink        transmission and does not expect any uplink transmissions;    -   g. If the AP initiated the TXOP in EDCA and is done with        downlink transmission and does not expect any further uplink        transmissions (for example with an End-of-Data signal (as shown        in FIG. 15), or a QoS-NULL frame with an ACK response from AP);    -   h. If the AP initiated the TXOP in HCCA with a CF-Poll and is        done with downlink transmission and does not expect any uplink        transmissions;    -   i. If the AP initiated the TXOP in HCCA with a CF-Poll and        receives an End-of-Data MAC signal from the STA, as shown in        FIG. 16, (or for example a QoS-NULL frame with an ACK response        from AP) and the AP is done with downlink transmission.

The multiple CF-End frames that are sent by the AP preferably observethe following rules individually or in combination:

-   -   a. The multiple CF-End frames will be sent only if they can be        sent before the current TXOP expires. This will be determined by        the AP by estimating the remainder of the TXOP and comparing to        the time required to send all CF-End frames;    -   b. If all the multiple CF-End frames cannot be sent before the        current TXOP expires only as many as can be sent before the        current TXOP expires will be sent;    -   c. In some cases or system conditions, even if all or some of        the multiple CF-End frames cannot be sent before the current        TXOP expires, they will all be still be sent even if some or all        of them have to be sent outside the TXOP.

The multiple CF-End frames sent by the AP enable all other devices inthe system to update their NAV and avoid potential waste or inefficiencyin medium usage. The multiple CF-End frames from the AP are separated bySIFS or other time duration, such as RIFS, as determined based on othersystem factors. The mechanism and order of transmission of the multipleCF-End frames (including dual CF-End frames if in a dual mode system)sent by the AP to release unused TXOP may be as follows depending on theoptions desired:

-   -   a. The multiple CF-End frames may be transmitted in an order of        priority as determined by the system configuration, which can        also be dynamically changed, where the priority corresponds to        that assigned to the modes supported by the system;    -   b. The first CF-End corresponds to the mode of the current TXOP        and the rest of the CF-End frames correspond to the other modes;    -   c. The order of the multiple CF-End frames corresponding to the        modes supported in the system may be arbitrary;    -   d. Just one CF-End frame in a format corresponding to the mode        of the current TXOP is sent in which case the medium is opened        up to all stations operating in that mode until the protection        for that mode expires, which gives a preference to the stations        operating in the mode of the TXOP;    -   e. If all stations can transmit and receive a single common mode        format—even if they normally communicate in a specific mode—that        common mode format should be used for sending one single CF-End        frame which will be sufficient to update the NAV of all stations        in all modes.

The following example is described with reference to a dual-mode systemapplication, where dual CF-End frames are in the ER(ExtendedRange)/NR(Normal Range) functionality, and where one CF-End frame issent in ER (STBC modulation), and the other CF-End frame is sent in NR(non-STBC modulation). The following describes one possibleimplementation of this dual CF-End frame example. If dual CTS protectionis enabled (i.e., STBC & non-STBC CTS frames sent by the AP when dualCTS protection is enabled in the system, typically indicated in thebeacon) and a STA obtains a TXOP and then the STA runs out of frames totransmit, then the STA may indicate “End of transmission” or “End ofdata” or “Truncation of its TXOP” by transmitting one of the followingframes, provided that the remaining TXOP duration will allow it (i.e.,that there is enough usable TXOP duration remaining after the CF-Endframes for release):

-   -   Case 1: A CF-End frame with the modulation that the STA is using        (STBC or non-STBC).    -   Case 2: A QoS-Null frame with the modulation that the STA is        using (STBC or non-STBC).    -   Case 3: Any other type of MAC frame that indicates “end of        transmission” or “end of data” signal—essentially indicating        that the STA has no more frames to send.

With the transmission of any one of the above indication frames (theabove Cases 1 to 3) the STA explicitly indicates the completion ortruncation of its TXOP. When the transmitted frame is a CF-End frame(Case1) it shall be interpreted by the other STAs that are capable ofreceiving it as a NAV reset.

On receiving any one of the above mentioned frames (the above Cases 1 to3) from a STA with a matching BSSID, an AP shall respond with dualCF-End frames—one STBC CF-End frame and one non-STBC CF-End frame—aftera SIFS duration (or other time duration, such as RIFS, as determinedbased on other system factors). Another possibility is that, in Case 2and any other frame that expects an ACK, the AP may first respond withan ACK before sending the dual CTS frames. Dual CF-End frames eliminateunfairness towards STAs that are not of the same mode as the one thatowns the TXOP being truncated.

If the TXOP is owned by the AP and dual CTS Protection is enabled in thesystem (usually indicated in the beacon i.e when both STBC and non-STBCSTAs are present in the system), the AP may send dual CF-End frames ifit runs out of frames to transmit provided that the remaining TXOPduration will allow it.

Further, in general when dual CTS Protection is enabled in the system asindicated in the beacon (i.e., when both STBC and non-STBC STAs arepresent in the system), the AP shall send dual CF-End frames—one STBCCF-End frame and one non-STBC CF-End frame—to do a NAV reset. STAs thatare capable of both modes may transmit dual CF-End frames when they wantto truncate their TXOPs if the remaining TXOP duration will allow it.

The spacing between the dual CF-End frames sent by the AP shall be SIFSor other time duration, such as RIFS, as determined based on othersystem factors. The order of frames in the dual CF-End frames may bearbitrary or one of them may be chosen to be sent first. In a firstpossible embodiment, the first CF-End frame shall use the samemodulation used for transmissions in the TXOP being truncated and thesecond CF-End frame shall use the other modulation. In other words, fora STBC TXOP the first CF-End is in STBC mode and for a non-STBC TXOP thefirst CF-End is in non-STBC mode.

Note that the solution above has both benefits of increased mediumutilization efficiency and elimination of unfairness towards STAs thatare not of the same mode as the one that owns the TXOP being truncated.This is because the CF-End sent by the owner of the TXOP to truncate theTXOP cannot be interpreted by the STAs of other modes and they willtherefore not be able to access the medium until the AP sends the dualCF-End (or multiple CF-End in the general case). Also the above solutionapplies in general to the case of a system with several modes (more thantwo).

The following describes a particular embodiment according to the aboveCases 1 to 3 that specifically applies to the 802.11n standardsspecification. If dual CTS protection is enabled and a STA obtains aTXOP and then the STA runs out of frames to transmit, the STA may thenindicate truncation of its TXOP, by transmitting a CF-End frame providedthat the remaining TXOP duration will allow it. For example, thiscondition may be determined according to the following determination:whether the remaining duration of the TXOP is greater than the sum ofCF-End frame duration, a STBC CF-End frame duration, a non-STBC CF-Endframe at a known basic rate, and two SIFS duration. With a CF-End frametransmission, the STA explicitly indicates the completion or truncationof its TXOP. The transmission of a CF-End frame shall be interpreted asa NAV reset by the other STAs that are capable of receiving it. Onreceiving a CF-End frame from a STA with a matching BSSID, an AP shallrespond with dual CF-End frames after SIFS duration—one STBC CF-Endframe and one non-STBC CF-End frame. If the TXOP is owned by the AP anddual CTS Protection is enabled in the system, the AP may send dualCF-End frames if it runs out of frames to transmit provided that theremaining TXOP duration will allow it. The spacing between the dualCF-End frames sent by the AP shall be SIFS. The first CF-End frame shalluse the same modulation used for transmissions in the TXOP beingtruncated and the second CF-End frame shall use the other modulation. Inother words, for a STBC TXOP the first CF-End is in STBC mode and for anon-STBC TXOP the first CF-End is in non-STBC mode.

The following describes another solution or mechanism which is simple inthat there is no need to send a dual CF-End but is less efficient inmedium utilization. When an STA or AP obtains a TXOP and uses the LongNAV mechanism to protect the TXOP duration, a CF-End frame is sent whenthere are no more frames to be sent indicating truncation or completionof TXOP. Our simplified solution is essentially to change the currentrules for TXOP truncation under Long NAV protection by disallowingsending of a CF-End frame by the owner of the TXOP when dual CTSProtection is enabled in the system (preferably indicated in thebeacon). So under these conditions, the TXOP will not be truncated bythe owner even if it has no more frames to send. This also applies ingeneral to the case of a system with several modes (more than two).

A STA, on receiving the CF_End frame (or MPDU) with a modulationcorresponding to its mode, can update its NAV (e.g., reset its NAV to 0)as follows:

-   -   a. The station updates its NAV after verifying that the BSSID        corresponds to its BSS (i.e. the BSS controlled by the AP with        which the STA is associated). If the BSSID does not match, the        STA does not update its NAV.    -   b. In some cases or implementations, the STA updates its NAV        regardless of the BSSID in the CF-End frame.

A third preferred embodiment of the invention defines a multiple modePSMP sequence for a multiple mode system. The PSMP sequence of prior artis designed to operate for a single mode. So to apply the prior art PSMPsequence in a multiple mode system, each mode would begin with dualCTS-to-Self frames followed by a PSMP frame and the scheduled downlinkand uplink transmissions. This procedure would have to be repeated foreach mode using the prior art PSMP sequence. This is not efficient usageof the medium and not flexible since multiple mode allocations cannot bemade in a single PSMP sequence.

FIG. 17 shows an example of the multiple mode PSMP sequence according tothe present invention. Here, the multiple mode PSMP sequence is definedas multiple mode CTS-to-self frames, followed by multiple mode PSMPframes, followed by the multiple mode downlink and uplink transmissions.Multiple mode PSMP frames define the schedule for the multiple modedownlink and uplink transmissions for the multiple mode PSMP sequenceduration. The multiple mode PSMP frames may define downlink time (DLT)allocations and uplink time (ULT) allocations of the stations in variousmodes in any order as determined suitable for applications andcapabilities of the devices and is completely flexible. Examples of theallocation ordering include, but are not limited to the following:

-   -   (1) all the downlink allocations of the same mode may be lumped        together—for example, there could be multiple STAs receiving in        the same mode and one STA in each DLT;    -   (2) all the uplink allocations of the same mode may be lumped        together;    -   (3) all uplink allocations are made after all the downlink        allocations (FIG. 17);    -   (4) the order of the STAs in the downlink allocations may be        preserved in the uplink allocations (FIG. 17).

Many other variants are possible on how the multiple mode PSMP framesmay define downlink time (DLT) allocations and uplink time (ULT)allocations. For example, a DLT can be followed by a ULT of the samemode. In other words, according to this third preferred embodiment, acompletely flexible ordering of ULT/DLT of any mode suitable for theapplications and capabilities of the devices is possible.

The multiple mode PSMP frames may be separated by PIFS or other timeduration, such as RIFS (Reduced Inter Frame Spacing), as determinedbased on other system factors.

The present invention may be implemented as a network having an accesspoint with multiple STAs or WTRUs, at the data link layer, medium accesscontrol, and network layer, as an application specific integratedcircuit (ASIC), digital signal processor (DSP) or software. The presentinvention relates to 802.11 based WLAN systems or OFDM/MIMO using radioresource management (RRM) and a radio resource controller (RRC).

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention. Themethods provided in the present invention may be implemented in acomputer program, software, or firmware tangibly embodied in acomputer-readable storage medium for execution by a general purposecomputer or a processor. Examples of computer-readable storage mediumsinclude a read only memory (ROM), a random access memory (RAM), aregister, cache memory, semiconductor memory devices, magnetic mediasuch as internal hard disks and removable disks, magneto-optical media,and optical media such as CD-ROM disks, and digital versatile disks(DVDs).

Suitable processors include, by way of example, a general purposeprocessor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one ormore microprocessors in association with a DSP core, a controller, amicrocontroller, Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs) circuits, any other type of integratedcircuit (IC), and/or a state machine.

A processor in association with software may be used to implement aradio frequency transceiver for use in a station (STA), wirelesstransmit receive unit (WTRU), user equipment (UE), terminal, basestation, radio network controller (RNC), or any host computer. The STAmay be used in conjunction with modules, implemented in hardware and/orsoftware, such as a camera, a video camera module, a videophone, aspeakerphone, a vibration device, a speaker, a microphone, a televisiontransceiver, a hands free headset, a keyboard, a Bluetooth® module, afrequency modulated (FM) radio unit, a liquid crystal display (LCD)display unit, an organic light-emitting diode (OLED) display unit, adigital music player, a media player, a video game player module, anInternet browser, and/or any wireless local area network (WLAN) module.

What is claimed is:
 1. A method for use in an access point (AP), themethod comprising: receiving a request-to-send (RTS) frame from astation (STA), wherein the RTS frame indicates a request for a transmitopportunity (TXOP); transmitting, in response to the RTS frame, anindication of a TXOP, wherein the indication of a TXOP is a space-timeblock code (STBC) clear-to-send (CTS) frame and a non-STBC CTS frame;receiving packet data from the STA during the TXOP; receiving, on acondition that the STA does not have further packet data to transmit, anindication of truncation of the TXOP including a first STBC contentionfree (CF)-End frame or a first non-STBC CF-End frame; and transmitting asecond STBC CF-End frame and a second non-STBC CF-End frame.
 2. Themethod of claim 1, wherein the receiving the indication of truncation ofthe TXOP occurs on a condition that sufficient TXOP duration isremaining.
 3. The method of claim 2, wherein the remaining TXOP durationis sufficient on a condition that the remaining TXOP duration is greaterthan a sum of a CF-End frame duration, an STBC CF-End frame duration, anon-STBC CF-End frame duration, and two short interframe spacing (SIFS)durations.
 4. The method of claim 1, wherein the indication oftruncation of the TXOP indicates a network allocation vector (NAV) resetto the AP.
 5. The method of claim 1, wherein each of the second STBCCF-End frame and the second non-STBC CF-End frame indicate an end of acontention free interval.
 6. The method of claim 1, wherein a shortinterframe spacing (SIFS) duration separates the second STBC CF-Endframe and the second non-STBC CF-End frame.
 7. The method of claim 1,wherein the receiving an indication of truncation of the TXOP includesreceiving a CF-End frame using a same modulation type as the receivedpacket data.
 8. The method of claim 1, wherein on a condition that theRTS frame is an STBC RTS frame, transmitting the STBC CTS frame followedby the non-STBC CTS frame, and on a condition that the RTS frame is anon-STBC RTS frame, transmitting the non-STBC CTS frame followed by theSTBC CTS frame.
 9. The method of claim 1, wherein on a condition thatthe indication of truncation of the TXOP is an STBC CF-End frame,transmitting the second STBC CF-End frame followed by the secondnon-STBC CF-End frame.
 10. The method of claim 1, wherein on a conditionthat the indication of truncation of the TXOP is a non-STBC CF-Endframe, transmitting the second non-STBC CF-End frame followed by thesecond STBC CF-End frame.
 11. An access point (AP) comprising: areceiver configured to receive a request-to-send (RTS) frame from astation (STA), wherein the RTS frame indicates a request for a transmitopportunity (TXOP); and a transmitter configured to transmit, inresponse to the RTS frame, an indication of a TXOP, wherein theindication of a TXOP is a space-time block code (STBC) clear-to-send(CTS) frame and a non-STBC CTS frame; wherein the receiver is furtherconfigured to receive packet data from the STA during the TXOP, andreceive, on a condition that the STA does not have further packet datato transmit, an indication of truncation of the TXOP including a firstSTBC contention free (CF)-End frame or a first non-STBC CF-End frame;and wherein the transmitter is further configured to transmit a secondSTBC CF-End frame and a second non-STBC CF-End frame.
 12. The AP ofclaim 11, wherein the receiver is further configured to receive theindication of truncation of the TXOP on a condition that sufficient TXOPduration is remaining.
 13. The AP of claim 12, wherein sufficient TXOPduration is remaining on a condition that a remaining TXOP duration isgreater than a CF-End frame duration, an STBC CF-End frame duration, anon-STBC CF-End frame duration, and two short interframe spacing (SIFS)durations.
 14. The AP of claim 11, wherein the receiver is furtherconfigured to receive an indication of truncation of the TXOP using aCF-End frame using a same modulation type as the received packet data.15. The AP of claim 11, wherein the indication of truncation of the TXOPindicates a network allocation vector (NAV) reset to the AP.
 16. The APof claim 11, wherein each of the second STBC CF-End frame and the secondnon-STBC CF-End frame indicate an end of a contention free interval. 17.The AP of claim 11, wherein the transmitter is configured to transmitthe second STBC CF-End frame and the second non-STBC CF-End frameseparated by a short interframe spacing (SIFS) duration.
 18. The AP ofclaim 11, wherein on a condition that the RTS frame is an STBC RTSframe, the transmitter is further configured to transmit the STBC CTSframe followed by the non-STBC CTS frame, and on a condition that theRTS frame is a non-STBC RTS frame, the transmitter is further configuredto transmit the non-STBC CTS frame followed by the STBC CTS frame. 19.The AP of claim 11, wherein on a condition that the indication oftruncation of the TXOP is an STBC CF-End frame, the transmitter isfurther configured to transmit the second STBC CF-End frame followed bythe second non-STBC CF-End frame.
 20. The AP of claim 11, wherein on acondition that the indication of truncation of the TXOP is a non-STBCCF-End frame, the transmitter is further configured to transmit thesecond non-STBC CF-End frame followed by the second STBC CF-End frame.